1. Field of the Invention
The present invention relates to a half bridge switching regulator configured by an output voltage adjusting switch element for adjusting an output voltage, and a synchronous switch element, connected in series with the output voltage adjusting switch element, that is complementary ON operated when the output voltage adjusting switch element is turned OFF.
2. Description of the Related Art
In this type of switching regulator, a soft switching method is adopted in which the output voltage adjusting switch element for adjusting the output voltage and the synchronous switch element that is complementary ON operated when the output voltage adjusting switch element is turned OFF are connected in series between input voltage terminals (in step-down regulator) or between output voltage terminals (in step-up regulator), and the synchronous switch element and the output voltage adjusting switch element are complementary ON/OFF operated by a switch control section to reduce switching loss and noise.
Japanese Laid-Open Patent Publication No. 07-46853 proposes a controlling method of performing, in a half bridge inverter configured by two switching elements, zero voltage switching and zero current voltage switching by providing a dead time with respect to the switching elements when switching each switching element ON/OFF as a soft switching inverter controlling method for reducing switching noise and switching loss and for controlling the output voltage by a constant switching frequency.
As shown in FIG. 1A, for instance, in a step-up switching regulator configured by an output voltage adjusting switch element Q1 for adjusting the output voltage and a synchronous switch element Q2 connected in series with the output voltage adjusting switch element Q1, the OFF timing of the synchronous switch element Q2 is determined by a map calculated value set, which is set in advance based on an input/output voltage values, to appropriately adjust the dead time.
This will be described in detail below. The step-up switching regulator is configured by the output voltage adjusting switch element Q1 for adjusting an output voltage Vout at an output voltage terminal OUT; a synchronous switch element Q2, connected in series with the output voltage adjusting switch element Q1, that is complementary ON operated when the output voltage adjusting switch element Q1 is turned OFF; a resonance circuit including a step-up coil L1 and a capacitor C1 that LC resonate in an aim of reducing the voltage Vds between the switch elements, which will be described later; and a bypass capacitor C2 for stabilizing the output voltage.
The output voltage adjusting switch element Q1 and the synchronous switch element Q2 are configured by n-channel MOS-FET, and a switch control section for switching each switch element ON/OFF by controlling the respective gate voltages of the output voltage adjusting switch element Q1 and the synchronous switch element Q2 is arranged.
The switch control section controls the switching regulator according to a timing chart shown in FIG. 1B. The switch control section determines a timing (TA1) to switch the output voltage adjusting switch element Q1 from ON to OFF based on the output voltage Vout. When the output voltage adjusting switch element Q1 is turned OFF, the voltage Vds between the switch elements rises, and the synchronous switch element Q2 is switched from OFF to ON at a timing (TA2) the voltage Vds between the switch elements and the output voltage Vout become equal.
The switch control section calculates the timing at which coil current IL, flowing through the step-up coil L1, that is reduced by turning ON the synchronous switch element Q2 becomes zero through map calculation, to be hereinafter described, and switches the synchronous switch element Q2 from ON to OFF at a timing (TA3) the calculated coil current IL becomes zero.
The output voltage adjusting switch element Q1 is switched from OFF to ON at a timing (TA4) the voltage Vds between the switch elements becomes zero. The switch control section controls the output voltage to be constant by repeating the basic operation described above.
In the basic operation described above, the timing of switching the synchronous switch element Q2 from ON to OFF, that is, the ON time t2 of the synchronous switch element in FIG. 1B can be obtained by [Eq. 1].
The maximum current Imax flowing through the step-up coil L1 in [Eq. 1] is obtained by [Eq. 2] based on the input voltage Vin at an input voltage terminal IN and time t0 defined by the OFF timing of the output voltage adjusting switch element Q1.
Therefore, the ON time t2 of the synchronous switch element is defined by the input voltage Vin and the output voltage Vout based on [Eq. 3] derived from [Eq. 1] and [Eq. 2].
                              t          2                =                              L            ·                          I              max                                                          V              out                        -                          V                              i                ⁢                                                                  ⁢                n                                                                        [                  Eq          .                                          ⁢          1                ]                                          I          max                =                                            V                              i                ⁢                                                                  ⁢                n                                      ·                          t              0                                L                                    [                  Eq          .                                          ⁢          2                ]                                          t          2                =                                            V                              i                ⁢                                                                  ⁢                n                                      ·                          t              0                                                          V              out                        -                          V                              i                ⁢                                                                  ⁢                n                                                                        [                  Eq          .                                          ⁢          3                ]            
The switch control section includes map data of the output voltage Vout with respect to various input voltages Vin created in advance, and the ON time t2 of the synchronous switch element is calculated based on the map calculation of applying the input voltage Vin and the output voltage Vout obtained from the map data to [Eq. 3].
A complicating circuit must be built to detect the input/output voltage or perform the map calculation when controlling the OFF timing of the synchronous switch element based on the result of the map calculation, but a feedback control is not used, and thus the OFF timing of the synchronous switch element cannot be controlled at satisfactory accuracy.
Furthermore, the map data must be set so that the synchronous switch element is turned OFF before the current backflows from the output side to the power supply side to ensure safety of the circuit, and thus the switching noise and the switch loss cannot be sufficiently reduced.